28 



fixed tenant of growing masses of white coral, but requiring at the same 

 time to be in relation with the surrounding element, it alters its spiral plan 

 of growth upon the increase of the coral, and pursues a nearly straight 

 course, in order to keep pace with the advancing surface of its rocky bed. 

 The Ianthina or Sea-Snail, is also an animal of peculiar habits, floating on 

 the bosom of the ocean by means of a number of albuminous air-bladders 

 attached to the foot, after the manner of a float ; and the parasitic habit of 

 the Stylifer, living on the juices of the Star-Fish, is curious. 



The shell of the Gastropods is either of a spiral, or of a simple conical 

 structure ; the spiral series is by far the more numerous in species, but it 

 does not include any of such weight or dimensions as the Giant Clam (Tri- 

 dacna gigas) among the Bivalves. Neither among the fossil shells of the 

 extinct species are there any so large as in the preceding class. The form 

 of the non-spiral gastropods varies from the depressly flattened cone of the 

 Umbrella to the extreme conical elevation of Denlalhim ; in one genus, 

 Chiton, the shell consists of eight distinct pieces moving upon each other 

 within a cartilaginous frame, like plate armour ; and in other genera, such as 

 Aplysia, the shell is merely represented by a concealed horny plate, depo- 

 sited by an internal fold of the mantle. In tins group is also contained an 

 interesting series called Nuclibranchiata, or naked-gilled mollusks, in winch 

 the gills are exposed in the most beautiful varieties of tufted and ramified 

 structure along the back : these have no shell.* The Limpet tribe afford 

 a curious phenomenon in the peculiarity of two of the genera, Litheda/phiis 

 and Hipponyx, depositing a shelly plate at their base of attachment. 



In treating of the parts of the spiral univalve shell it may be observed, 

 in the first place, that the typical structure, of which all the different 

 generic forms are modifications, is that of an enlarging conical tube, winding 

 obliquely from left to right (viewing the mollusk as moving forward from 

 the observer) by reason, probably, of some peculiar winding tendency in 

 the vital organs of the animal. The axis upon which the tube winds, is 

 called the axial pillar or columella ; every turn around tins axis is called a 

 whorl ; and when the columella is hollow, it is said to be umbilicated. 

 Among the terrestrial Gastropods the most simple plan of convolution is 

 exhibited in the Cyclostoma g'igantewm ; for an elongated modification of 

 tins, the Pupa Ruschenbergiana may be quoted as a remarkable example, 

 the diameter of the tube in tins shell is very small, and the volutions are per- 

 formed witlun a very limited area ; and there being as many as five and twenty 

 whorls the shell assumes the form of a peculiarly attenuated cylinder. Tor 

 a depressed modification of the univalve, the Caracolla parmula may be re- 

 garded as a most characteristic example ; in this shell the spiral tube is so 

 depressly flattened as to present the form of a slightly convex lens. In the 



* Although the Nudlbranchial Mollusks furnish no ornaments for the cabinet and do not 

 enter, therefore, into the category of animals treated of in this work, I cannot refrain from 

 noticing Alder and Hancock's beautiful figures of them recently published by the Ray Society. 



